A pneumatic wheel and tyre assembly, when inflated, is an acoustically resonant toroidal structure with, in the case of automobile wheels, a cavity frequency typically around 200 Hz. Resonant vibration results from exciting a standing wave in the air-filled torus by driving the automobile over a coarse road surface, for example of concrete or asphalt containing large stones. The resultant cavity noise may be transmitted to the interior of the vehicle by either airborne or structure-borne transfer paths. The amplitude of the noise may be sufficient to cause discomfort to the vehicle's passengers, and may cause a reduction in the perceived quality of the vehicle.
Tyre cavity noise may be controlled by modifying the forcing function, transfer functions or response functions in the vehicle. The most effective intervention typically is in the forcing function i.e. in the tyre cavity itself. To this end it is known to introduce a cavity noise absorber into a pneumatic tyre which attenuates the cavity resonance either by dividing the cavity space to prevent the formation of a standing wave, or by absorbing the energy of a standing wave in an acoustic damping material.
Conventional tyre noise absorbers suffer a number of disadvantages. Some are mounted to the inside of the circumferential wall of a tyre. In this position they interfere with the deployment of liquid puncture repair systems, which are now increasingly provided in place a spare wheel for automobiles to reduce both cost and vehicle mass. Others are mounted to the outside of the wheel, and in this position they can interfere with the process of fitting a tyre to the wheel. There are also problems with absorber durability and stability at high rotational speeds.
It is an object of embodiments of the present invention to address some or all of these problems.